1. Field of the Invention
The present invention relates to a continuous casting immersion nozzle for pouring molten steel from a tundish into a mold.
2. Description of the Related Art
In a continuous casting process for producing casting steel products of a predetermined shape by continuously cooling and solidifying molten steel, molten steel is poured into a mold through a continuous casting immersion nozzle (hereafter, also referred to as the “immersion nozzle”) positioned at the bottom of a tundish.
Generally, the immersion nozzle includes a tubular body with a bottom, and a pair of outlets. The tubular body has an inlet for entry of molten steel disposed at an upper end and a passage extending inside the tubular body downward from the inlet. The pair of outlets are disposed in the sidewall at a lower section of the tubular body so as to communicate with the passage. The immersion nozzle is used with its lower section submerged in molten steel in the mold to prevent flying of poured molten steel into the air and oxidation thereof through contact with the air. Further, the use of the immersion nozzle allows regulation of the molten steel flow in the mold and thereby prevents impurities floating on the molten steel surface such as slags and non-metallic inclusions from being entrapped into the molten steel.
In recent years, there has been a demand for improving the quality and productivity of steel in the continuous casting process. Increasing the productivity of steel with existing production facilities requires rising the pouring rate (throughput). Thus, in order to increase the amount of molten steel that passes through the immersion nozzle, attempts have been made through approaches such as increasing the diameter of the nozzle passage and increasing the dimensions of the outlets within a limited space in the mold.
Increasing the outlet dimensions results in imbalances in flow velocity distribution between the exit-streams discharged out of the lower portions and the exit-streams out of the upper portions of the outlets, and between the exit-stream out of the right outlet and the exit-stream out of the left outlet. The imbalanced flows (drifts) impinge on the narrow sidewalls of the mold and then induce unstable patterns of molten steel flow in the mold. As a result, the level fluctuation at the molten steel surface is caused by excessive reverse flows, and the steel quality is lowered due to entrapment of mold power, and also problems such as breakout occur.
International publication No. 2005/049249, for example, discloses an immersion nozzle including a tubular body, the body having a pair of opposing lateral outlets in the sidewall of a lower section thereof. The lateral outlets each are divided by one or two inward horizontal projections into two or three vertically arranged portions to make a total of four or six outlets (See FIGS. 18A and 18B). International publication No. 2005/049249 describes that the immersion nozzle permits inhibition of clogging and generation of more stable and controlled exit-streams which are more uniform in velocity and in which spin and swirl are significantly reduced.
The present inventors performed water model tests regarding the immersion nozzle of International publication No. 2005/049249, a conventional type immersion nozzle, and a modification of the conventional type immersion nozzle (See FIG. 19), to study variations in the pattern of molten steel flow from each immersion nozzle. The conventional type immersion nozzle includes a tubular body having a pair of opposing outlets in the sidewall at a lower section. The modified type immersion nozzle includes opposing ridges projecting into the passage from the inner surface of the immersion nozzle, the ridges disposed on the middle between the opposing outlets.
FIGS. 20A and 20B show graphs indicating the results of the water model tests regarding the immersion nozzles. In the graph of FIG. 20A, the abscissa represents the average value σav of the standard deviations of the velocities of the reverse flows on the right- and left-hand sides of the immersion nozzle as seen in a view showing the mold's broad sidewall in front, and the ordinate represents the difference Δσ between the standard deviations of the velocities of the right- and left-hand reverse flows. In the graph of FIG. 20B, the abscissa represents the average value σav of the standard deviations of the velocities of the right- and left-hand reverse flows, and the ordinate represents the average value Vav of the velocities of the right- and left-hand reverse flows. In addition, sample A corresponds to the immersion nozzle of International publication No. 2005/049249 (four-outlet type nozzle), sample B corresponds to the conventional type immersion nozzle, and sample C corresponds to the modified type immersion nozzle. FIG. 20A indicates that the conventional type immersion nozzle (sample B) exhibited the largest difference Δσ between the standard deviations of the velocities of the right- and left-hand reverse flows, namely, the largest difference between the velocities of the right- and left-hand reverse flows, while the immersion nozzle of International publication No. 2005/049249 (sample A) and the modified type immersion nozzle (sample C) exhibited smaller differences between the velocities of the right- and left-hand reverse flows. On the other hand, FIG. 20B indicates that the conventional type immersion nozzle (sample B) and the immersion nozzle of International publication No. 2005/049249 (sample A) exhibited larger average values Vav of the velocities of the right- and left-hand reverse flows and that the modified type immersion nozzle (sample C) exhibited the smallest average value Vav.
The difference Δσ between the standard deviations of the velocities of the right- and left-hand reverse flows and the average value Vav of the velocities of the right- and left-hand reverse flows increase with a rise in throughput. From the viewpoint of improving the quality of slabs, it is desirable that Δσ is 2 cm/sec or less, and that Vav is 10 cm/sec to 30 cm/sec. Note that Δσ of all the samples were 2 cm/sec or less, while Vav of all the samples were outside the range of 10 cm/sec to 30 cm/sec.
In the case of the immersion nozzle of International publication No. 2005/049249 (four-outlet type nozzle), as indicated by the results of the fluid analyses in FIGS. 21A, 21B, larger amounts of the exit-streams issued from the lower portions of the outlets while smaller amounts from the upper portions, with the result that the velocities of the reverse flows were as high as 35 cm/sec. For the fluid analyses, the mold was set to have dimensions of 1500 mm×235 mm and the throughput was set to 3.0 ton/min.
Further, the immersion nozzle of International publication No. 2005/049249, which has four or more outlets, not only requires a complicated manufacturing process, but is liable to induce imbalance between the right- and left-hand exit-streams when clogging or thermal wear of the outlets occurs.
The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an immersion nozzle for continuous casting which reduces the drift of molten steel flowing from the outlets of the nozzle and reduces the level fluctuation at the molten steel surface and which is easy to manufacture.